maybe two firewalls would be hard to hack (as in once the first one, passed, u got the second one) but you'd have to put different OS, or diff firewall and config b/c if it's the exact same (ie: two NT boxes, filterning the exact same traffic (same rules)) then if u hack the first one, you can hack the second.
So globally I believe more than one firewall/router DOES have a use, but u have to set it up "inteligent-ly"
something most admins tend to forget